The present invention relates to the field of nuclear magnetic resonance imaging systems (NMR), including, but not limited to, magnetic resonance imaging (MRI) devices. In particular, the present invention provides an apparatus and method for testing the dynamic response of such magnetic devices to a changing magnetic field.
Magnetic resonance imaging techniques generally employ pulsed magnetic field gradients to spatially encode nuclear magnetic resonance (NMR) signals from various portions of an anatomical or physical region of interest. The pulsed magnetic field gradients, together with radio frequency excitation of the nuclear spins, and acquisition of signal information are commonly referred to as a pulse sequence. Thus, NMR systems including MRI scanners, for instance, employ magnetic pulse sequences to create a visual image of a subject. In these sequences, magnetic fields are turned on and off, or otherwise changed at very rapid rates, to precisely calculated levels, all at precisely timed intervals.
However, the changing of magnetic fields produces inductive effects, including eddy currents. These inductive effects create their own opposing magnetic fields and thus delay and/or distort the desired magnetic fields required for precise imaging. The level of delay and distortion of the desired magnetic fields is called the dynamic response. Thus, a degraded dynamic response, that is a high level of delay and/or distortion, results in degraded performance by a particular NMR system. Such degradation in the dynamic response of the particular NMR system can be brought about by, for example, manufacturing and installation defects including continuous, closed, electrically conductive paths.
In the field of NMR systems, the dynamic response of the particular NMR systems, including, but not limited to MRI scanners, is detected and measured using H-B testing, which is a method of comparing the dynamic response of two different magnetic fields. The H-field is a magnetic field of a coil in free space, and the B-field is the magnetic field of another coil or set of coils that is/are not in free space, or in the present case within the gap of an iron frame magnet. The H-field follows along with the input current function, while the B-field is distorted due to eddy currents and hysteresis effects, when driven by the same input current function. Two pick-up coils that are wired together in series opposition, with one of the pick-up coils being placed in the H-field and the other pick-up coil being placed in the B-field, will give the H-B signal, which is the difference between the signal from the pick-up coil in the H-field and the signal from the pick-up coil in the B-field. In order to obtain H-B measurements, a testing apparatus requires an H-field producing coil that can handle substantial electrical currents, an H-field coil support that holds the coil away from any conductive or magnetic objects, H- and B- field pick-up coils that convert magnetic field changes into electrical impulses, an integrator that reconstructs the electrical impulses into a form that represents the magnetic field, an oscilloscope that facilitates signal capture and measurement, and a programmable current source that supplies the electric current to thereby produce both the H- and the B- fields.
Presently, H-B testing are performed only for particular components of an NMR system, and the testing apparatus has been adapted to fit the configuration of the particular system. Thus, excitation coils are required to be bolted or otherwise affixed to the NMR system component, and the center and rotation thereof must be aligned using plumb bobs, levels and rulers. The varying configurations of the NMR systems results in uncontrolled test data which is not appropriate for comparison with other NMR system configurations.
Furthermore, according to current methods for H-B testing of NMR system components as described above, when a next level of assembly of the NMR system is reached, the H-B test requires that the excitation coils that were bolted or otherwise affixed to a component in the previous stage of assembly and testing must be removed and mounted once again at a different distance from the poles. Such testing method is vulnerable to inconsistent H-B measurements, often resulting from the test method itself and the alignment of the testing apparatus.
Such inconsistencies in the alignment of the testing device relative to the poles of the MRI scanning device can actually result in dipoles, that is the effect being measured. Dipoles, or dynamic dipoles, occur when the magnetic fields from two sources that are designed to cancel each other are different as a function of time while they are changing. If the two excitation coils are misaligned, they may induce a different set of eddy currents in each pole and therefore there will be a net field in regions where the net field should be zero (0) until the eddy currents dissipate. Dynamic dipoles occur as the result of defects or other differences in the poles.
Therefore, an object of the present invention is to provide an apparatus and method that tests an NMR system, including MRI scanners, by taking H-B measurements for the apparatus at any stage during the manufacture thereof, to thereby detect any defects of the NMR system at any point during the manufacture thereof. Specifically, after each successive step in the assembly of the NMR system, the present invention provides verification that the immediately preceding assembly step did not affect the dipole and speed characteristics of the NMR system without incurring additional assembly or significant amounts of time.
Thus, another object of the present invention is to provide an apparatus and method that quantifies the dipole and speed characteristics of any particular set-up of an NMR system.
As set forth above, the present invention is directed to the field of nuclear magnetic resonance imaging systems (NMR). However, in order to provide a clear and practical understanding of the invention, the following summary and disclosure of the invention will be directed to a magnetic resonance imaging (MRI) scanning apparatus, although the present invention is clearly not so limited. That is, the present invention has application in all system embodiments within the field of nuclear magnetic resonance imaging (NMRI), and the following description is by way of example only.
Accordingly, to accomplish the objects described above and other objects as well, the present invention provides a self-contained apparatus, having parallel plates in the configuration of a double sided table, that can be inserted into a gap of the MRI scanner, or any other magnetic device, as the scanner is being assembled or even upon completion of the scanner. The self-contained apparatus that provides dipole and magnetic gap speed measurement in an MRI scanner or other magnetic device includes excitation coils, as described above, that are attached to each of the parallel plates.
However, to overcome at least the deficiencies described above associated with known H-B testing methods, the self-contained apparatus of the present invention includes a controlling mechanism that controls the distance between the fixed excitation coil and the polecap by providing consistent spacing between the excitation coils and the poles for every level of manufacture of the MRI scanner. To that end, the present invention further includes precision machined excitation coil mounts and a precision machined pin-centering mechanism that maintain upper and lower excitation coils in coaxial alignment with the poles; vertical height adjustment mechanisms that enable vertical alignment and planar leveling of the excitation coils relative to the poles of the MRI scanner by keeping the excitation coils a fixed distance apart from each other, keeping the mid-point of the vertical height adjustment mechanism coincident with the midpoint of the magnetic gap; and a pick-up coil holder/positioner mechanism and grid that enable quick and accurate positioning of the pick-up coils relative to the MRI scanner/magnetic device poles. The present invention is a self-contained apparatus that, because it may be preassembled in one-piece, may be quickly and efficiently inserted into the MRI scanner or other magnetic device at any stage of assembly thereof for testing.